NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: DR thread from Nov-Dec '04
From: Doug Royer
Date: 2005 Jan 17, 17:12 -0800
From: Doug Royer
Date: 2005 Jan 17, 17:12 -0800
Bill, My main source was being taught(made aware of)this in classes when I studied to sit for my ABS exam and later my 3rd mate exam.It was shown to me while still an ABS by more than 1 officer in actual situations while plying oceanic waters.It has served me well(as a rule of thumb)over the course of my sea going career.There are many veriables.Some of the resulting calculations get complex.As when the officer/helmsman have to hold the ship on track and calculate what I mentioned in the origonal post.Wind currents,leeway,oceanic currents,bow shear,screw torgue, screw slip and other variables must be vectored to obtain the desired corrections to stay close to the trackline.When engaged in commerce shipping time is money and money is time.So,getting this right is important as the captian or 1st officer wouldn't be pleased to find the vessel is x nm off the track line at the end of a watch,thus taking longer to get the cargo into port(it could be bad enough that the vessel would miss the correct tide and have to ride it out outside the harbor and enter the harbor on the next tide).Bad JuJu for the OOW if this happens! Some one(on this list)once mentioned that I must keep good or exceptional DRs when we were discussing some topic.Damn right I did!I didn't like getting chewed out by my superior officers for mistakes! This is a great rule of thumb and is suprisingly accurate when applied. There are a few books that study this and give examples but I've never seen it outside of a book not geared for the professional. "The Merchant Marine Officer's Handbook" 4th or 5th edition devotes space to it in the chapters dealing with shiphandling and pilotting.I haven't seen a 6th edition yet. Other than that one book perhaps in other marine teaching institution's sylabus as that is where I was 1st exposed to it. Doug I've read on the subject in several different texts but had little blue water opportunity to actually try it. Some add variables such as fetch, velocity, and how long the wind has been coming from the same direction (minimum 12 hrs?) to questimate the velocity of the current and how far off the wind direction it is shifted (up to 45d) by the Coriolis Effect. Wondered what your source(s) is, and if it/they mentions the above fuzzy variables? Thanks Bill